I. Field
The present invention relates generally to communication, and more specifically to techniques for supporting soft handoff in a wireless frequency hopping communication system using interference cancellation.
II. Background
In a frequency hopping communication system, data is transmitted on different frequency subbands in different time intervals, which may be referred to as “hop periods”. These frequency subbands may be provided by orthogonal frequency division multiplexing (OFDM), other multi-carrier modulation techniques, or some other constructs. With frequency hopping, the data transmission hops from subband to subband in a pseudo-random manner. This hopping provides frequency diversity and allows the data transmission to better withstand deleterious path effects such as narrow-band interference, jamming, fading, and so on.
An orthogonal frequency division multiple access (OFDMA) system utilizes OFDM and can support multiple users simultaneously. For a frequency hopping OFDMA system, a data transmission for a given user may be sent on a “traffic” channel that is associated with a specific frequency hopping (FH) sequence. This FH sequence indicates the specific subband to use for the data transmission in each hop period. Multiple data transmissions for multiple users may be sent simultaneously on multiple traffic channels that are associated with different FH sequences. These FH sequences may be defined to be orthogonal to one another so that only one traffic channel, and thus only one data transmission, uses each subband in each hop period. By using orthogonal FH sequences, the multiple data transmissions do not interfere with one another while enjoying the benefits of frequency diversity.
An OFDMA system may be deployed with multiple cells, where a cell typically refers to the coverage area of a base station. A data transmission on a given subband in one cell acts as interference to another data transmission on the same subband in a neighboring cell. To randomize inter-cell interference, the FH sequences for each cell are typically defined to be pseudo-random with respect to the FH sequences for neighboring cells. By using pseudo-random FH sequences for different cells, interference diversity is achieved and a data transmission for a user in one cell observes the average interference from the data transmissions for other users in other cells.
In a multi-cell OFDMA system, it is desirable to support “soft handoff”, which is also referred to as “soft handover”. Soft handoff is a process whereby a user communicates with multiple base stations simultaneously. Soft handoff can provide spatial diversity against deleterious path effects via transmission of data to or from multiple base stations at different locations. However, soft handoff is complicated when the system employs frequency hopping. This is because the FH sequences for one cell are pseudo-random (i.e., not orthogonal) with respect to the FH sequences for neighboring cells in order to randomize inter-cell interference. A user in soft handoff with multiple base stations may be instructed to use an FH sequence by a designated base station among the multiple base stations. The data transmission sent by the soft-handoff user will be orthogonal to the data transmissions sent by other users of the designated base station, but will be pseudo-random with respect to the data transmissions sent by users of other base stations. The soft-handoff user would cause interference to the users of the other base stations and would also receive interference from these users. The interference degrades the performance of all affected users, unless it is mitigated in some manner.
There is therefore a need in the art for techniques to support soft handoff in a frequency hopping OFDMA system.